The present invention relates to plastisols in general and in particular to improved adhesion promoter systems for polyvinyl chloride (PVC) plastisols.
Vinyl chloride homopolymers (PVC) and copolymers are widely used for the protection of metallic surfaces against corrosion, for adhering thin sheet metal constructions, and for sealing welded seams, particularly in the automotive industry. Such coatings principally are applied in the form of plasticized vinyl chloride polymers (plastisols) by a variety of techniques. Depending upon the viscosity of the PVC plastisol, its utilization can be characterized as a sealant, caulk, coating, adhesive, or other function.
Since resistance to corrosion is a prime prerequisite in dealing with metal parts, the PVC plastisol should adhere to the metal part with a fair degree of tenacity; otherwise, oxidation undercutting would result. Despite the ability to control the texture of the plastisol by suitable fillers and the color by the addition of suitable tinctorial pigments, of the PVC plastisol must be overcoated with a high performance, e.g. acrylic, topcoat, especially in automotive uses of the PVC plastisol. Uniformity in visual appearance dictates such overcoating requirement typically.
Another requirement of the PVC plastisol is that it be curable at short, low bake cycles. In fact, successful plastisols and organisols in the automotive industry are being called on today to be curable, dry to the touch, at baking temperatures of less than 300.degree., e.g. about 275.degree. F., with oven residence times of about 20-30 minutes. Energy costs dictate such low temperature, short baking cycles. Further, successful plastisol and organisol formulations must be economic and expel a minimum of atmospheric pollutants.
While a variety of adhesion promoters have been proposed for PVC plastisols and organisols, most have suffered from a variety of drawbacks including cost, insufficient film flexibility, or the like. One class of adhesion promoters which appears to exhibit a good balance between cost, flexibility, and low temperature bake characteristics are polyaminoamide-polyimidazoline adhesion promoters, such as set forth in U.S. Pat. No. 4,146,520. Such adhesion promoters have enabled the formulation of advantageous PVC plastisols and organisols which can be baked at temperatures as low as about 250.degree. F. Unfortunately, the plastisols and organisols containing such polyaminoamide-polyimidazole adhesion promoters are not readily overcoated with acid-catalyzed topcoats, especially high performance acid catalyzed acrylic automobile finishes. It is theorized that the amine value of the adhesion promoters interacts and renders ineffectual the acid catalyst in the topcoat, thus resulting in tacky films. While reduction of the level of the adhesion promoter can overcome such topcoat tackiness, loss of adhesion also can occur at such low adhesion promoter levels. Thus, there is a need in the art to enable the utilization of polyaminoamide-polyimidazoline adhesion promoters in low bake PVC plastisol and organisol compositions, yet provide the ability to overcoat such plastisols with acid catalyzed topcoats.